Refrigeration System And Method For Operating A Refrigeration System

ABSTRACT

A refrigeration system ( 2 ) comprises a condenser/gas cooler ( 4 ), an intermediate expansion device ( 6 ) and a refrigerant collecting container ( 8 ); a normal refrigeration branch ( 10 ) connecting the refrigerant collecting container ( 8 ) to the condenser/gas cooler ( 4 ) said normal refrigeration branch ( 12 ) comprising a first expansion device ( 12 ), a first evaporator ( 14 ) and a compressor unit ( 16 ) of the normal refrigeration branch ( 10 ); a freezing branch ( 18 ) connecting the refrigerant collecting container ( 8 ) to the condenser/gas cooler ( 4 ), said freezing branch ( 18 ) comprising a second expansion device ( 20 ), a second evaporator ( 22 ), and a first compressor unit ( 24 ) and a second compressor unit ( 26 ) of said freezing branch ( 18 ), the first and second compressor units ( 24, 26 ) of the freezing branch ( 18 ) being connected in series. A flash gas line ( 28 ) is provided connecting the gas space of the refrigerant collecting container ( 8 ) to the line connecting the first compressor unit ( 24 ) to the second compressor unit ( 26 ) of said freezing branch ( 18 ). Refrigerant conduits for connecting said elements and for circulating a refrigerant therethrough are also provided.

The invention relates to a refrigeration system and a method ofoperating a refrigeration system.

Conventional refrigeration systems, for instance in supermarkets,typically include both a medium temperature refrigeration salesfurniture, wherein for example food items such as fruits, vegetables,and beverages are stored and cooled, and a low temperature/freezingsales furniture where food items are stored and kept in a frozencondition.

From the WO 2006015741 a refrigeration circuit is known employing amedium temperature compressor set and a low temperature compressor set.The refrigeration circuit further comprises in the direction of flow, acondenser, a collector, a pressure-relief device, arranged before anevaporator, an evaporator and a singlestage compressor unit. Anintermediate pressure-relief device is arranged between thecondenser/gas cooler and the collector. Furthermore, a method foroperating such refrigeration circuit is disclosed.

It would be beneficial to provide an alternative, efficientrefrigeration system providing both low temperature and mediumtemperature refrigeration.

Exemplary embodiments of the invention include a refrigeration systemcomprising a condenser/gas cooler, an intermediate expansion device anda refrigerant collecting container and a normal refrigeration branchconnecting the refrigerant collecting container to the condenser/gascooler, the normal refrigeration branch comprising a first expansiondevice, a first evaporator and a compressor unit of the normalrefrigeration branch. The refrigeration system further comprises afreezing branch connecting the refrigerant collecting container to thecondenser/gas cooler, the freezing branch comprising a second expansiondevice, a second evaporator, and a first and a second compressor unit ofthe freezing branch, the first and second compressor units of thefreezing branch being connected in series. Moreover, a flash gas line isprovided connecting the gas space of the refrigerant collectingcontainer to the line connecting the first compressor unit to the secondcompressor unit of the freezing branch. Further, the refrigerationsystem comprises refrigerant conduits for connecting said elements andfor circulating a refrigerant therethrough.

Moreover, exemplary embodiments of the invention include a method foroperating a refrigeration system comprising a condenser/gas cooler, anintermediate expansion device and a refrigerant collecting container,the method comprising the steps of: operating a normal refrigerationbranch between the refrigerant collecting container to the condenser/gascooler, the normal refrigeration branch comprising a first expansiondevice, a first evaporator and a compressor unit of the normalrefrigeration branch; operating a freezing branch between therefrigerant collecting container to the condenser/gas cooler, thefreezing branch comprising a second expansion device, a secondevaporator, and a first compressor unit and a second compressor unit ofthe freezing branch, the first and second compressor units of thefreezing branch being connected in series, and feeding flash gas fromthe gas space of the refrigerant collecting container to the lineconnecting the first compressor unit to the second compressor unit ofthe freezing branch by means of a flash gas line.

Exemplary embodiments of the invention will be described in greaterdetail below taking reference to the accompanying drawings.

FIG. 1 shows a schematic view of a refrigeration system according toexemplary embodiment of the invention.

FIG. 2 shows an embodiment of an oil management system in therefrigeration system of FIG. 1.

FIG. 3 shows a further embodiment of an oil management system in therefrigeration system of FIG. 1.

FIG. 4 shows a further embodiment of an oil management system in therefrigeration system of FIG. 1.

FIG. 5 shows a further embodiment of an oil management system in therefrigeration system of FIG. 1.

FIG. 6 shows a further embodiment of an oil management system in therefrigeration system of FIG. 1.

FIG. 7 shows a further embodiment of an oil management system in therefrigeration system of FIG. 1.

FIG. 8 shows a further embodiment of an oil management system in therefrigeration system of FIG. 1.

The refrigeration system 2 shown in FIG. 1 comprises, in flow directionof a refrigerant circulating therein, a gas cooler/condenser 4, anintermediate pressure expansion device 6, a refrigerant collectingcontainer 8 as well as a normal refrigeration branch 10 and a freezingbranch 18 arranged in parallel.

In the condenser/gas cooler 4, the refrigerant is cooled down against asecondary medium. In the exemplary embodiment of FIG. 1, the secondarymedium is air as for example ambient air of the outside of asupermarket. Other secondary media, such as water, sole or air withwater particles, may also be used. In the case of CO₂ being therefrigerant and the refrigeration system being operated in thetranscritical mode, the condenser/gas cooler 4 is referred to as gascooler, as the refrigerant leaves the condenser/gas cooler 4 in gaseousphase. For other refrigerants and/or a sub-critical operation, acondensation takes place in the condenser/gas cooler 4, such that thisrefrigerant circuit element is referred to as condenser. In this case,the relatively warm refrigerant enters the gas cooler/condenser 4 ingaseous phase, is cooled down below the dew-point of the refrigerant andleaves the gas cooler/condenser 4 in liquid phase.

The intermediate pressure expansion device 6 located between the gascooler/condenser 4 and the refrigerant collecting container 8 expandsthe high pressure refrigerant, that has been cooled by the condenser/gascooler 4 to an intermediate pressure.

The refrigerant as it leaves the condenser/gas cooler 4 has a typicalpressure level from 10 to 120 bar (10*10⁵ to 120*10⁵ Pa), particularly60 to 100 bar (60*10⁵ to 100*10⁵ Pa) and more particularly 80 bar(80*10⁵ Pa) and is then expanded to an intermediate pressure of 5 to 40bar (5*10⁵ to 40*10⁵ Pa), particularly 25 to 35 bar (25*10⁵ to 35*10⁵Pa) and more particularly 30 bar (30*10⁵ Pa). By using an intermediatepressure expansion device 6, the lines and the equipment downstream theintermediate pressure expansion device 6 can be designed for a lowerpressure to reduce costs and increase efficiency of the refrigerationsystem 2. The pressure of the refrigerant after it has been expanded inthe intermediate expansion device 6 is still below theevaporation-pressure of the refrigerant, so that it remains in theliquid phase.

As can be seen in FIG. 1, the refrigerant line coming from theintermediate pressure expansion device 6 connects to an upper portion ofthe refrigerant collecting container 8, that has a lower liquid spaceportion, in which liquid refrigerant collects and an upper gaseousrefrigerant portion, in which gaseous refrigerant collects.

The refrigeration system 2 further comprises a normal refrigerationbranch 10 and a freezing branch 18 arranged in parallel.

The refrigerant line coming from the refrigerant collecting container 8splits up into the normal refrigeration branch 10 and the freezingbranch 18 both connecting the refrigerant collecting container 8 to thecondenser/gas cooler 4.

The normal refrigeration branch 10 comprises in flow direction of therefrigerant, a first expansion device 12, a first evaporator 14 and acompressor unit 16. In the present non-limiting embodiment thecompressor unit 16 consists of three compressors arranged in parallel.

In the first expansion device 12 of the normal refrigeration branch 10the refrigerant is further expanded to a pressure between 30 and 35 bar(30*10⁶ to 35*10⁶ Pa), particularly 32 bar (32*10⁶ Pa), and therebycooled down to a desired refrigeration temperature between −1 and 5° C.(272.15 and 278.15K).

Downstream of the first expansion device 12 of the normal refrigerationbranch 10, there is provided a first evaporator 14 in which therefrigerant is heated against the environment, which thereby is cooled.

The evaporators 14 and 22 as described in the invention can be referredto as medium and low temperature cold consumers that can be formed asrefrigerating furnitures, like refrigerating cabinets, refrigeratingchests or refrigerating islands, or as refrigerating rooms, that providegoods arranged therein with the desired refrigeration. The firstevaporator 14 of the normal refrigeration branch 10 can for example be anormal temperature food refrigerator used in a supermarket to cool downfoods to a temperature from 0 to 5° C. (273.15 to 278.15K).

The heated refrigerant exits the first evaporator 14 in a gaseous phaseand flows to the suction side of the first compressor unit 16 of thenormal refrigeration branch 10, in which the gaseous refrigerant iscompressed to high pressure and led back to the gas cooler/condenser 4.

The freezing branch 18 connects the refrigerant collecting container 8to the condenser/gas cooler 4 and comprises in flow direction of therefrigerant, a second expansion device 20, a second evaporator 22, afirst compressor unit 24 and a second compressor unit 26. The first andthe second compressor units 24 and 26 of the freezing branch 18 areconnected in series.

Similar to the normal refrigeration branch 10, the liquid refrigerant inthe freezing branch 18 is expanded in the second expansion device 20 andevaporated in the second evaporator 22 arranged downstream of the secondexpansion device 20. Compared to the normal refrigeration branch 10, therefrigerant is expanded to a lower pressure in the second expansiondevice 20 in a range of 10 to 20 bar (10*10⁵ to 20*10⁵ Pa), particularly15 bar (15*10⁵ Pa), to achieve a lower temperature in the secondevaporator 22 of approximately −20 to −40° C. (253.15 to 233.15K),particularly −30° C. (243.15K).

The second evaporator 22 of the freezing branch 18 can for example be afreezer to freeze foods to a temperature up to −40° C. (233.15K).

Leaving the second evaporator 22 in a gaseous phase the refrigerantflows to the suction side of the first compressor unit 24 of thefreezing branch 18 consisting of three compressors arranged in parallel,in the present non-limiting embodiment of the invention.

In the first compressor unit 24 of the freezing branch 18, the gaseousrefrigerant is compressed to a medium pressure level below or equal tothe pressure of the line entering the gas cooler/condenser 4.

Downstream of the first compressor unit 24 of the freezing branch 18,there is provided a second compressor unit 26 consisting in the presentnon-limiting embodiment of two compressors arranged in parallel. Thesecond compressor unit 26 of the freezing branch 18 is referred to as abooster compressor that is frequency controlled to match the mass flowin line 34 connecting the first and second compressor units 24 and 26.In the second compressor unit 26 the refrigerant is compressed to a highpressure level complying to the pressure at the exit of the compressorunit 16 of the normal refrigeration branch 10. A typical pressure levelat the high pressure side of the system between the second compressorunit 26 of the freezing branch 18, the compressor unit 16 of the normalrefrigeration branch 10 and the intermediate expansion device 6 is 20 to120 bar (20*10⁵ to 120*10⁵ Pa), particularly 50 to 100 bar (50*10⁵ to100*10⁵ Pa) and more particularly 80 bar (80*10⁵ Pa).

Leaving the second compressor unit 26 of the freezing branch 18, thegaseous refrigerant enters the gas cooler/condenser 4 together with thegaseous refrigerant coming from the compressor unit 16 of the normalrefrigeration branch 10.

The compression of the gaseous refrigerant of the freezing branch 18 isaccomplished in two stages consisting of medium stage compression in thefirst cornpressor unit 24 and a high compression in the secondcompressor unit 26.

According to a discovery of the inventor, the two-stage compression ofthe refrigerant of the freezing branch 18 leads to a lower number ofcompressors needed to compress the refrigerant from a low pressure levelat the outlet oft the second evaporator 22 to a high pressure level asneeded at the inlet of the gas cooler/condenser 4. The number ofcompressors that can be reduced by the refrigeration system of thepresent invention comprises the compressors building the first andsecond compressor units 24 and 26 of the freezing branch 18. The numberof compressors depicted in the Fig. is only of exemplary kind.

Attaching to the upper gas space portion of the refrigerant collectingcontainer 8 and leading to the line 34 connecting the first and secondcompressor units 24 and 26 of the freezing branch 18, there is provideda flash gas line 28 by means of which flash gas from the refrigerantcollecting container 8, in particular from its upper gas space portioncan be sucked off by the compressor unit 26.

The lower liquid space of the refrigerant collecting container isconnected to the normal refrigeration branch 10 and to the freezingbranch 18 to ensure that only liquid refrigerant is supplied to theexpansion devices 12 and 20 and to the evaporators 14 and 22.

By connecting the refrigeration collecting container 8 to the suctionside of the second compressor unit 26 of the freezing branch 18, thepressure of the refrigerant collecting container 8 is controlled by thepressure of the second compressor unit 26. Hence, the exemplaryrefrigeration system does not require an economizer stage comprisingadditional regulating valves to control the medium and high dischargepressure, as used in conventional refrigeration systems. That results inlower vibration of the present refrigeration system due to less valvesinstalled in the system that have to be opened and closed.

Moreover, the pressure of the refrigerant collecting container 8 can becontrolled more precisely which increases the overall system efficiency.A more constant pressure of the refrigerant collecting container 8 alsoreduces instabilities at the evaporators 14 and 22 and expansion valveslevels 12 and 20 allowing for a more stable overall systemcharacteristic.

FIG. 2 shows an embodiment of an oil management system 31 in arefrigeration system 2 as described in FIG. 1. A simplified detail ofthe refrigeration system 2 of FIG. 1 is shown in FIG. 2, wherein onlythe compressor units consisting of the compressor unit 16 of the normalrefrigeration branch 10 and the first and second compressor units 24 and26 of the freezing branch 18 and the lines around those elements can beseen.

The first and second compressor units 24 and 26 of the freezing branch18 are connected by the line 34 which is also connected to the gas spaceof the refrigerant collecting container 8 via the flash gas line 28. Theflash gas line 28 connecting the gas space of the refrigerant collectingcontainer 8 to the line connecting the first compressor unit 24 to thesecond compressor unit 26 of the freezing branch 18 only containsgaseous refrigerant that does not contain any oil.

In FIG. 2, there is provided an oil balance line 30 equipped with avalve 32, for example a solenoid valve 32 connecting the oil sumps ofthe compressor unit 10 of the normal refrigeration branch 10 to oilsumps of the first compressor unit 24 of the freezing branch 18. Bymeans of this oil balance line 30, the oil levels within the oil sumpsof the compressor unit 16 of the normal refrigeration branch 10 and theoil sumps of the first compressor unit 24 of the freezing branch 18 canbe balanced.

The valve 32 can for example be pressure controlled so as to open andclose the oil balance line 30 at a certain pressure difference betweenthe oil sumps of the compressor unit 16 of the normal refrigerationbranch 10 and the oil sumps of the first compressor unit 24 of thefreezing branch 18. If the solenoid valve is switched open, the oil fromthe oil sumps of the compressor unit 16 is fed to the oil sumps of thefirst compressor unit 24 by the pressure difference, and by theincreased oil rate within the refrigerant in the first compressor unit24 and the refrigerant exiting the same, the oil level in the secondcompressor unit 26 will be raised as well.

In operation of the exemplary refrigeration system 2 oil generallycollects at the compressor unit 16 of the normal refrigeration branch 10whereas the compressor units 24 and 26 of the freezing branch 18 are notsufficiently supplied with oil. Especially, the oil rate of therefrigerant at the inlet of the second compressor unit 26 of thefreezing branch 18 is relatively low due the fact that flash gas, notcontaining any oil, from the gas space of the refrigerant collectioncontainer 8 is led to the inlet of the second compressor unit 26.

The valve 32 can further be controlled by a control unit (not shown)monitoring the oil levels in the oils sumps of the first and secondcompressor units 24 and 26 of the freezing branch 18 and the compressorunit 16 of the normal refrigeration branch. The valve 32 can be switchopen by the control unit if the oil level of one of the oil sumps of thefirst or second compressor units 24 and 26 falls below a predeterminedvalue or if the oil level of the oil sumps of the compressor unit 16 ofthe normal refrigeration branch exceeds a predetermined value.Alternatively or additionally, the valve 32 can be switched open atpredetermined time intervals independently from the oil levels in theoil sumps of the compressors 16, 24 and 26. The time interval can bedifferent for every solenoid valve and can depend on the change of theoil levels over the time in each sump.

By an oil balance line 30 of the present embodiment, low oil levels ofthe oil sumps of the first and second compressor units 24, 26 of thefreezing branch 18 and too high oil levels of the oil sumps of thecompressor unit 16 of the normal refrigeration branch 10 are reliablyavoided and therefore defects of those compressors being caused byinsufficient lubrication or by oil hits can be considerably reduced. Thevalve 32 can be controlled to fed the oil either continuously orintermittently to the respective oil sumps and allows to adjust the oilflow rate depending on the oil demand of the compressors.

FIG. 3 shows an embodiment of an oil management system 33 in arefrigeration system 2 as described in FIG. 1. A simplified detail ofthe refrigeration system 2 of FIG. 1 is shown in FIG. 3, wherein onlythe compressor units consisting of the compressor unit 16 of the normalrefrigeration branch 10 and the first and second compressor units 24 and26 of the freezing branch 18 and the lines around those elements can beseen.

An oil balance line 35 connects the oil sumps of the compressing unit 16of the normal refrigeration branch 10 to the suction side of the firstcompressor unit 24, particularly to the suction line of the firstcompressor unit 24 at a point before it divides up into the separatelines leading to each of the compressors of the compressor unit 24. Theoil balance line 35 is provided with a valve 32, for example a solenoidvalve 32.

By means of this oil balance line 35, the oil levels within the oilsumps of the cornpressor unit 16 of the normal refrigeration branch 10and the oil rate of the first compressor unit 24 of the freezing branch18 can be balanced.

The valve 32 can for example be pressure controlled so as to open andclose the oil balance line 30 at a certain pressure difference betweenthe oil sumps of the compressor unit 16 of the normal refrigerationbranch 10 and the suction side of the first compressor unit 24 of thefreezing branch 18. If the valve 32 is switched open, the oil from theoil sumps of the compressor unit 16 is fed to the suction side of thefirst compressor unit 24 by the pressure difference, and by theincreased oil rate within the refrigerant in the first compressor unit24 and within the refrigerant exiting the same, the oil level in thesecond compressor unit 26 will be raised as well.

In operation of the exemplary refrigeration system 2 oil generallycollects at the compressor unit 16 of the normal refrigeration branch 10whereas the compressor units 24 and 26 of the freezing branch 18 are notsufficiently supplied with oil. Especially, the oil rate of therefrigerant at the inlet of the second compressor unit 26 of thefreezing branch 18 is relatively low due the fact that flash gas, notcontaining any oil, from the gas space of the refrigerant collectioncontainer 8 is led to the inlet of the second compressor unit 26.

The valve 32 can further be controlled by a control unit (not shown)monitoring the oil rates of the first and second compressor units 24 and26 and, respectively, the oil levels in the oils sumps of the first andsecond compressor units 24 and 26 of the freezing branch 18 and thecompressor unit 16 of the normal refrigeration branch. The valve 32 canbe switched open by the control unit if the oil level of one of the oilsumps of the first or second compressor units 24 and 26 falls below apredetermined value or if the oil level of the oil sumps of thecompressor unit 16 of the normal refrigeration branch exceeds apredetermined value. Alternatively or additionally, the valve 32 can beswitched open at predetermined time intervals independently from the oillevels in the oil sumps of the compressors 16, 24 and 26. The timeinterval can be different for every valve and can depend on the changeof the oil levels over the time in each sump.

By an oil balance line 35 of the present embodiment, low oil levels ofthe oil sumps of the first and second compressor units 24, 26 of thefreezing branch 18 and too high oil levels of the oil sumps of thecompressor unit 16 of the normal refrigeration branch 10 are reliablyavoided and therefore defects of those compressors being caused byinsufficient lubrication or by oil hits can be considerably reduced.

The valve 32 can be controlled to feed the oil either continuously orintermittently and allows to adjust the oil flow rate depending on theoil demand of the compresSors.

FIG. 4 shows an embodiment of an oil management system 36 in arefrigeration system as described in FIG. 1, comprising the elementsalready shown in FIG. 2, as the first and second compressor units 24, 26of the freezing branch 18 and the compressor unit 16 of the normalrefrigeration branch 10 and the conduits 34 and 28.

As can be seen in FIG. 3 an oil balance line 38 connects the oil sumpsof the compressor unit 16 of the normal refrigeration branch 10 to theoutlet of the first cornpressor unit 24 of the freezing branch 18. Theoil balance line 38 further comprises a valve 40 by means of which theoil balance line 38 can be switched open or closed. To balance the oilflow between the compressor unit 16 of the normal refrigeration branch10 and the outlet of the compressor unit 24 of the freezing branch 18,the valve 40 is for example controlled by the oil level in the oil sumpsof the second compressor unit 26 of the freezing branch 18. If the oillevel in the oil sumps of the second compressor unit 26 falls below acertain value, the valve 40 is switched open to allow oil from the oilsumps of the compressor unit 16 to flow to the outlet of the secondcompressor unit 26.

In a further embodiment, the valve 40 is controlled by the oil level inthe first compressor units 24 of the freezing branch 18. To avoid a toolow oil level in the oil sumps of the first compressor unit 24, thevalve 40 is opened if the oil level of the oil sumps of the firstcompressor unit 24 falls below a certain value to feed oil to the outletof the compressor unit 24 which is connected to the suction side of thecompressor unit 26 by the line 34.

Another possibility of balancing the oil comprises detecting the oilrate in the line 34 connecting the first and second compressor units 24and 26 of the freezing branch 18. By monitoring the oil rate in the line34 and opening the valve if the oil rate falls below a certain value,sufficient oil supply to the compressor unit 26 is ensured.

An oil balance line 38 as described in this embodiment is especiallybeneficial if the compressor unit 24 is always sufficiently suppliedwith oil. Therefore, oil from the compressor unit 16 is supplied to thecompressor unit 26 only due to the fact that the flash gas is fed to theinlet thereof.

A further embodiment of an oil management system 42 in a refrigerationsystem 2 is shown in FIG. 5 which also depicts the compressor units 26,24 and 16, wherein the first and second compressor units 24 and 26 ofthe freezing branch 18 are connected by the line 34 to which the flashgas line 28 is connected.

In the embodiment shown in FIG. 5, there is provided an oil balance line44 comprising a valve 46 and connecting the oil sumps of the compressorunit 16 of the normal refrigeration branch 10 to the oil sumps of thefirst compressor unit 24 of the freezing branch 18. An additional oilbalance line 48 comprising an additional valve 50 branches off the oilbalance line 44 and connects the oil sumps of the compressor unit 16 ofthe normal refrigeration branch 10 to the oil sumps of the secondcompressor unit 26 of the freezing branch 18.

For the oil balancing and re-feeding operation the valves 46 and 50 canboth be switched open to allow an oil flow from the oil sumps of thecompressor unit 16 to the oil sumps of the first and second compressorunits 24 and 26 of the freezing branch 18.

The valves 46 and 50 can for example be controlled by the pressure ofthe oil sumps of the first and second compressor units 24 and 26 so asto allow for an oil flow to said oil sumps in case the pressure of theoil sumps falls below a certain value.

Further, the valves 46 and 50 can be controlled by the pressuredifference between the oil sumps of the compressor units. If for examplethe pressure difference between the oil sumps of the compressor unit 16and the oil sumps of the compressor unit 24 exceed a certain value, thevalve 46 is switched open. That also applies to the valve 50 which isswitched open if the pressure difference between the oil sumps of thecompressor unit 16 and the oil sumps of the compressor unit 26 exceeds acertain value.

A benefit of this embodiment is that the valves 46 and 50 and therebythe oil flow in the oil balance lines 44 and 48 can be controlledindependently so as to ensure an oil flow from the oil sumps of thecompressor unit 16 to the oil sumps of the compressor units 24 and 26depending on their individual oil demand.

FIG. 6 shows a further embodiment of an oil management system 52comprising every element already shown in FIG. 5, except for the oilbalance line 44 and the corresponding valve 46. In contrast to FIG. 5,an oil balance line 54 comprising a valve 56 is connected to the outletof the first compressor unit 24 of the freezing branch 18.

Thereby, the oil is fed directly into the gaseous refrigerant in line 34connecting the first compressor unit 24 to the second compressor unit 26of the freezing branch 18.

A further embodiment of an oil management system 58 in a refrigerationsystem 2 is shown in FIG. 7 which also depicts the compressor units 26,24 and 16, wherein the first and second compressor units 24 and 26 ofthe freezing branch 18 are connected by the line 34 to which the flashgas line 28 is connected. The embodiment in FIG. 6 comprises an oilbalance line 60 connecting the oil sumps of the compressor unit 16 ofthe normal refrigeration branch 10 to the oil sumps of the compressorunit 24. Furthermore, an oil balance line 64 is provided in theembodiment of FIG. 5 connecting the oil sumps of the compressor unit 16to the outlet of the first compressor unit 24. Each of the oil balancelines 60 and 64 comprises a valve 62 and 66 respectively. Controllingthe valves 62 and 66 can be performed depending on the parametersalready mentioned in the description of the preceding figures as forexample the pressure difference between the oil sumps of the compressorunit 16 and the oil sumps of the compressor unit 24 and/or compressorunit 26, the pressure in the oil sumps of the compressor units 24 and 26and the oil rate in the refrigerant in line 34.

FIG. 8 shows a further embodiment of an oil management system 68containing every element described in FIG. 6 and an additional oilbalance branch 70 with an additional valve 72 arranged therein. The oilbalance branch 70 connects the oil sumps of the second compressor unit26 of the freezing branch 18 to the oil balance branches 60 and 64leading to the oil sumps and the outlet of the first compressor unit 24of the freezing branch 18 and to the oil sumps of the compressor unit 16of the normal refrigeration branch 10.

In the embodiment of FIG. 8, three locations at which a lack of oil canoccur are connected to the oil sump of the compressor unit 16 of thenormal refrigeration branch. By measuring the oil content in therefrigerant in line 34 and the oil levels of the oil sumps of the firstand second compressor units 24 and 26 of the freezing branch 18 andcontrolling the valves 62, 66 and 72 accordingly, oil can be directed toeach of the three locations depending on their individual demand.

While the refrigerating circuit and the corresponding method accordingto exemplary embodiments, as described above, is generally suitable fora wide variety of refrigerants, carbondioxide (CO₂) is particularly wellsuited.

According to an exemplary embodiment, as described above with respect toFIG. 1, the two-stage compression of the refrigerant of the freezingbranch leads to a lower number of compressors needed to compress therefrigerant from a low pressure level at the outlet of the secondevaporator to a high pressure level as needed at the inlet of the gascooler/condenser. Moreover, the exemplary refrigeration system does notrequire an economizer stage comprising additional regulating valves tocontrol the medium and high discharge pressure, as used in conventionalrefrigeration systems. That results in lower vibration of the presentrefrigeration system due to less valves installed in the system thathave to be opened and closed.

Further, a more constant pressure of the refrigerant collectingcontainer is achieved, which also reduces instabilities at theevaporators and expansion valves levels and allows for a more stableoverall system characteristic.

By exemplary embodiments of the invention, as described with respect toFIGS. 2 to 8, the oil levels of the medium temperature compressors ofcompressor unit and the low temperature compressors of compressor unitsare balanced automatically. The excess amount of oil that regularlycollects at the oil sumps of the compressor unit of the normalrefrigeration branch is automatically fed to the low temperaturecompressor side, in particular to the oil sumps of the second compressorunit and if applicable also to the first compressor unit of the freezingbranch. Thereby, too low oil levels at the freezing temperaturecompressor side and too high oil levels at the normal refrigerationtemperature compressor side are reliably avoided and therefore defectsof those compressors being caused by insufficient lubrication can beconsiderably reduced.

According to an embodiment, the refrigeration system further comprisesan oil balance line connecting at least one oil sump of the compressorunit of the normal refrigeration branch to at least one of thecompressors of the first and second compressor units of the freezingbranch.

Thereby, it is ensured that oil can flow from the compressor unit of thenormal refrigeration branch to at least one of the compressors of thefirst and second compressor units of the freezing branch. The inventorhas discovered that normally the first and second compressors of thefreezing branch tend to have a too low oil level in their oil sumps andthat especially the second compressor of the freezing branch is notsufficiently supplied with oil due to the fact that the flash gas streamcoming from the refrigerant collecting container does not contain anyoil so that the combined refrigerant flow at the suction side of thecompressor unit does not contain sufficient oil for lubrication of thecompressor unit.

According to an embodiment, the oil balance line connects at least oneoil sump of the compressor unit of the normal refrigeration branch to atleast one oil sump of the first compressor unit of the freezing branch.

Thus, an oil flow to the sumps of the first compressor unit of thefreezing branch is allowed which leads to sufficient oil supply thereof,as well as a higher oil rate in the discharge gas which, combined withthe flash gas, is led to the second compressor unit of the freezingbranch. Hence, both the first and second compressor units are suppliedwith oil.

According to an embodiment, the oil balance line connects at least oneoil sump of the compressor unit of the normal refrigeration branch tothe suction side of the first compressor unit of the freezing branch.

Thus, an oil flow to the suction side of the first compressor unit ofthe freezing branch is allowed which leads to sufficient oil supplythereof, as well as a higher oil rate in the discharge gas which,combined with the flash gas, is led to the second compressor unit of thefreezing branch. Hence, both the first and second compressor units aresupplied with oil.

According to an embodiment, the oil balance line can be switched open orclosed by means of at least one valve, for example a solenoid valvearranged therein.

Since the oil level of the oil sumps of the compressor units in thesystem depend on varying system parameters, as for example ambienttemperature and refrigerating capacity, oil does not need to be fedcontinuously to the first and second compressor units of the freezingbranch. A valve arranged in the oil balance line allows to adjust theoil flow rate to the demand for oil of the compressor units.

According to an embodiment, the refrigeration system further comprises acontrol unit configured to switch the valve in the oil balance line openif the oil level in the second compressor unit of the freezing branchfalls below a predetermined threshold value.

Monitoring the oil level in the second compressor unit, where a lack ofoil most likely occurs, ensures sufficient oil supply since the oil flowrate can be adjusted to the oil level in the oil sumps.

According to an embodiment, the refrigeration system, further comprisesa control unit configured to switch a valve in an oil balance line openif the oil level in the first compressor unit of the freezing branchfalls below a predetermined threshold value. Thereby, a too low oillevel in the first compressor unit of the freezing branch is avoided andthe oil flow rate can be adjusted to the oil demand of thereof.

According to an embodiment, the refrigeration system further comprises acontrol unit configured to switch a valve in an oil balance line open ifthe oil level in the compressor unit of the normal refrigeration branchexceeds a predetermined threshold value.

In operation, oil generally collects in the oil sumps of the compressorunit of the normal refrigeration branch. To avoid a too high oil levelin the oil sumps of the compressor unit, a monitoring device is arrangedin the oil sumps to measure the oil level thereof and transmit a signalto the control unit if the oil level exceeds a predetermined level.Thereby, the oil sumps of the compressor unit can be designed forsmaller volumes and additional elements as for instance overflow valvesand refrigerant collectors are not necessary.

According to an embodiment, the refrigeration system comprises a controlunit configured to switch a valve in an oil balance line open if the oillevel in the compressor unit of the normal refrigeration branch exceedsa predetermined threshold value and/or if the oil level in the secondcompressor unit of the freezing branch falls below a predeterminedlevel. Hence, the valve is controlled by two parameters and the oil flowrate can be adjusted depending on the oil levels in both the compressorunits.

According to an embodiment, the refrigeration system comprises a controlunit configured to switch a valve in an oil balance line open if the oillevel in the compressor unit of the normal refrigeration branch exceedsa predetermined threshold value and/or if the oil level in the secondcompressor unit of the freezing branch falls below a predetermined leveland/or if the oil level in the first compressor unit of the freezingbranch falls below a predetermined threshold value.

In this embodiment the valves in the oil balance lines connecting thecompressor units of the refrigeration system can be controlled by acontrol unit to which paw rameter values concerning the oil level or oilpressure in the oil sumps or the oil content in the refrigerant aretransmitted. The control unit can switch the valves open or close toallow an oil flow through the respective oil balance line leading to theoil sump where oil is needed.

According to an embodiment, a valve is configured to be opened after apredetermined time interval. Thus, there are no monitoring devices andcontrol units needed in the refrigeration system to adjust the oil flowto the demand of the respective compressor units.

This embodiment offers a more simple and cost efficient way to feed oilintermittently to the oil sumps of the compressor units. The timeinterval, at which the valves are opened or closed, has to comply to thechange of the oil levels in the oil sumps over the time for a certainoperation condition of the refrigeration system. The time intervals canbe different for every valve to control the oil flow in the respectiveoil balance line.

According to an embodiment, an oil balance line connects the at leastone oil sump of the compressor unit of the normal refrigeration branchto an outlet of the first compressor unit of the freezing branch, saidoutlet being part of the first cornpressor unit.

This results in a higher oil content in the line connecting the firstand the second compressor unit of the freezing branch. Thus, sufficientoil supply to the second compressor unit of the freezing branch isensured.

According to an embodiment, the oil balance line further connects atleast one oil sump of the second compressor unit of the freezing branchto the at least one oil sump of the compressor unit of the normalrefrigeration branch and the at least one of the compressors of thefirst compressor unit of the freezing branch.

In this embodiment all of the compressor units of the refrigerationsystem are connected to allow an oil exchange between the compressorunits in case of too high or too low oil levels in any of the compressorunits.

According to an embodiment, the oil balance line connects the at leastone oil sump of the compressor unit of the normal refrigeration branchand the at least one oil sump of the second compressor unit of thefreezing branch to at least one oil sump of the first compressor unit ofthe freezing branch.

Thereby, the oil sumps of all the compressor units in the refrigerationsystem are connected to each other and oil may be fed to any of the oilsumps of the compressor units depending on their demand.

According to an embodiment, the oil balance line connects the at leastone oil sump of the compressor unit of the normal refrigeration branchand the at least one oil sump of the second compressor unit of thefreezing branch to an outlet of the first compressor unit of thefreezing branch.

In case of an oil-shortage in the second compressor unit of the freezingbranch oil can be supplied directly to the oil sumps of the compressorunit and to the outlet of the first compressor unit of the freezingbranch so that the oil in the refrigerant flowing from the first to thesecond compressor unit is enriched with oil. That results in a higheroil content of the combined refrigerant flow from the refrigerantcollecting container and from the first compressor unit at the inlet ofthe second compressor unit. This embodiment ensures an adequate oillevel in the oil sumps and a sufficient oil content of the refrigerantat the inlet of the second compressor unit simultaneously.

According to an embodiment, the second compressor unit of the freezingbranch is frequency controlled to match the mass flow from the firstcompressor unit of the freezing branch and from the flash gas line.

This arrangement offers a more stable and energy efficient refrigerationsystem compared to the standard system. Since the pressure of therefrigerant collecting container is controlled by the frequency andhence by the capacity of the second compressor unit of the freezingbranch, this pressure can be controlled more precisely. As a result, theoverall system efficiency is increased and a more constant pressure inthe refrigerant collecting container is achieved which reducesinstabilities at the evaporators and the expansion valves connected tothe refrigerant collecting container, allowing for a more stable overallsystem characteristic.

According to an embodiment, the refrigerant is CO₂ or a CO₂ blend.

Compared to conventional refrigerants CO₂ has important environmentalfriendly characteristics as for example being non-flammable andnon-ozone depleting. Its physical properties are highly favourable forcooling, refrigeration and heating purposes, having a high volumetriccooling capacity. Due to its operation at pressures of up to 130 bar(130·10⁵ Pa), systems require highly resistant components that have beenalready developed to serial production in many sectors.

According to an embodiment, the first evaporator and the secondevaporator are cooling devices of a supermarket.

According to an embodiment, the normal refrigeration branch is parallelto the freezing branch and the compressor unit of the normalrefrigeration branch is parallel to the first and second compressorunits of the freezing branch.

Thereby, a low temperature freezing branch and a normal refrigerationtemperature branch are realized wherein the discharge of the compressorunit of the normal refrigeration temperature branch is not led to theinlet of the first and second compressor units of the freezingtemperature branch. Moreover, the proposed systern does not require aneconomizer stage as used in standard refrigeration systems which leadsto lower vibration due to the fact that less valves are needed tocontrol the discharge of the refrigerant collecting container.

The automatic oil balancing and oil re-feeding provided by theparticular embodiments, as described above, can be realized easily andcost-efficiently for all booster systems no matter what performance. Theproposed refrigeration system has been shown to be more energy efficientcompared to the standard system especially at ambient conditions above30° C. (300.15K) which is because the amount of flash gas increases athigh temperatures and the standard systems. The proposed system canreduce the annual energy consumption by 1% to 4% below the energyconsumption of the standard system.

Additionally, thermodynamic losses occurring in the standard system byusing an economizing stage, can be prevented in the present invention.Furthermore, in the proposed system, the compressor unit of the normalrefrigeration branch requires a smaller suction volume than thecompressor in the standard system which also leads to a smaller totalsuction volume of the system. Hence, the number of compressors used inthe proposed system can be reduced.

All the advantages and the embodiments that have been described withrespect to the refrigerating circuit also hold true for thecorresponding method. These advantages and embodiments are herewithexplicitly disclosed also in terms of corresponding method steps,however without repeating them again.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt the particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore it is intendedthat the invention not be limited to the particular embodimentsdisclosed, but that the invention will include all embodiments fallingwithin the scope of the appended claims.

1. Refrigeration system, comprising: a condenser/gas cooler, anintermediate expansion device and a refrigerant collecting container; anormal refrigeration branch connecting the refrigerant collectingcontainer to the condenser/gas cooler, said normal refrigeration branchcomprising a first expansion device, a first evaporator and a compressorunit of the normal refrigeration branch; a freezing branch connectingthe refrigerant collecting container to condenser/gas cooler, saidfreezing branch comprising a second expansion device, a secondevaporator, and a first compressor unit and a second compressor unit ofsaid freezing branch, the first and second compressor units of thefreezing branch being connected in series, wherein a flash gas line isprovided connecting the gas space of the refrigerant collectingcontainer to the line connecting the first compressor unit to the secondcompressor unit of said freezing branch; further comprising refrigerantconduits for connecting said elements and for circulating a refrigeranttherethrough.
 2. Refrigeration system according to claim 1, furthercomprising an oil balance line connecting at least one oil sump of thecompressor unit of the normal refrigeration branch to at least one ofthe compressors of the first and second compressor units of the freezingbranch.
 3. Refrigeration system according to claim 2, wherein the oilbalance line connects at least one oil sump of the compressor unit ofthe normal refrigeration branch to at least one oil sump of the firstcompressor unit of the freezing branch.
 4. Refrigeration systemaccording to claim 2, wherein the oil balance line connects at least oneoil sump of the compressor unit of the normal refrigeration branch tothe suction side of the first compressor unit of the freezing branch. 5.Refrigeration system according to claim 2, wherein said oil balance linecan be switched open or closed by means of at least one valve arrangedtherein.
 6. Refrigeration system according to claim 2, comprising acontrol unit configured to switch the valve in the oil balance line openif the oil level in the second compressor unit of the freezing branchfalls below a predetermined threshold value.
 7. Refrigeration systemaccording to claim 2, comprising a control unit configured to switch thevalve in the oil balance line open if the oil level in the firstcompressor unit of the freezing branch falls below a pre-determinedthreshold value.
 8. Refrigeration system according to claim 2,comprising a control unit configured to switch the valve in the oilbalance line open if the oil level in the compressor unit of the normalrefrigeration branch exceeds a predetermined threshold value. 9.Refrigeration system according to claim 2, comprising a control unitconfigured to switch the valve in the oil balance line open if the oillevel in the compressor unit of the normal refrigeration branch exceedsa predetermined threshold value.
 10. Refrigeration system according toclaim 2, comprising a control unit configured to switch the valve in theoil balance line open if the oil level in the first compressor unit ofthe freezing branch falls below a predetermined threshold value. 11.Refrigeration system according to claim 2, wherein the valve isconfigured to be opened after a predetermined time interval. 12.Refrigeration system according to claim 2, wherein the oil balance lineconnects at least one oil sump of the compressor unit of the normalrefrigeration branch to an outlet of the first compressor unit of thefreezing branch.
 13. Refrigeration system according to claim 2, whereinthe oil balance line connects at least one oil sump of the compressorunit of the normal refrigeration branch to at least one oil sump of thesecond compressor unit of the freezing branch and to at least one of thecompressors of the first compressor unit of the freezing branch. 14.Refrigeration system according to claim 2, wherein the oil balance lineconnects to at least one oil sump of the first compressor unit of thefreezing branch.
 15. Refrigeration system according to claim 2, whereinthe oil balance line connects to an outlet of the first compressor unitof the freezing branch.
 16. Refrigeration system according to claim 1,wherein the second compressor unit of the freezing branch is frequencycontrolled to match the mass flow from the first compressor unit of thefreezing branch and from the flash gas line.
 17. Refrigeration systemaccording to claim 1, wherein the refrigerant is C02.
 18. Refrigerationsystem according to claim 1, wherein said first evaporator and saidsecond evaporator are located in refrigerating sales furnitures orcooling/freezing rooms of a supermarket.
 19. Refrigeration systemaccording to claim 1, wherein the normal refrigeration branch isparallel to the freezing branch.
 20. Refrigeration system according toclaim 1, wherein the compressor unit of the normal refrigeration branchis parallel to the first and second compressor units of the freezingbranch.
 21. Method for operating a refrigeration system, comprising: acondenser/gas cooler, an intermediate expansion device and a refrigerantcollecting container, the method comprising: operating a normalrefrigeration branch between the refrigerant collecting container to thecondenser/gas cooler, said normal refrigeration branch comprising afirst expansion device, a first evaporator and a coin pressor compressorunit of the normal refrigeration branch; operating a freezing branchbetween the refrigerant collecting container to the condenser/gascooler, said freezing branch comprising a second expansion device, asecond evaporator, and a first compressor unit and a second compressorunit of said freezing branch, the first and second compressor units ofthe freezing branch being connected in series, and feeding flash gasfrom the gas space of the refrigerant collecting container to the lineconnecting the first compressor unit to the second compressor unit ofsaid freezing branch by means of a flash gas line.